1. Field of the Invention
This invention relates generally to telecommunication systems, and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
The transmission power used to transmit symbols over an air interface, e.g., the air interface between a mobile unit and a base station (or a base station and a mobile unit) in a wireless telecommunication system, is typically determined by balancing competing constraints. For example, increasing the transmission power may reduce a bit error rate associated with transmissions over the air interface. However, increasing the transmission power may also increase system interference between mobile units and/or base stations. Increasing the transmission power of a transmitter in a mobile unit may also reduce the operational lifetime of a battery in the mobile unit. Thus, the transmission power may be selected to balance the competing desires for a low bit error rate, low system interference, long battery life, and increased system capacity. In order to maintain a proper balance between these competing factors, it is desirable to maintain a constant received power at the receiver.
The path loss between the transmitter and the receiver in a mobile wireless environment varies with time. This time varying nature is caused by fading inherent to a terrestrial wireless channel (commonly referred to as fast fading), or due to distance between the transmitter and receiver, or due to shadow obstructions. Thus under proper control, the optimal transmission power typically varies over time to compensate for the time-varying path loss. For example, the transmission power of a transmitter in a mobile unit and/or a base station may be continually increased to maintain an acceptable bit error rate as the mobile unit moves away from the base station, even though this may result in reduced battery life and/or increased system interference. Thus, closed loop power control systems may be used to control the transmission power of transmitters in wireless telecommunication systems. In a typical closed loop power control system, such as may be implemented in wireless telecommunications systems that operate in accordance with TDMA, IEEE 802.xx, CDMA 2000 and/or UMTS protocols, feedback information is sent from a receiver to a transmitter at regular intervals. For example, the transmission power associated with slots in a frame may be controlled using a single power control bit that may be transmitted once per slot. The power control bit may instruct the transmitter to either reduce or increase the transmitted power for the next slot, depending upon a signal-to-noise ratio associated with a received signal, such as a pilot or traffic signal.
In one conventional technique, the value of the power control bit is determined by comparing a signal-to-noise ratio associated with a received signal to a reference value, sometimes referred to as a set point. For example, a ratio of the signal energy to a noise power spectral density (Ec/Nt) for a received pilot or traffic signal may be compared to a reference value of Ec/Nt. The power control bit may then be set to instruct the transmitter to reduce the transmitted power when Ec/Nt is larger than the reference value and to increase the transmitted power when Ec/Nt is less than the reference value. For another example, the chip energy (Ec) for a received pilot or traffic signal may be compared to a reference value of the chip energy Ec. The power control bit may then be set to instruct the transmitter to reduce the transmitted power when Ec is larger than the reference value and to increase the transmitted power when Ec is less than the reference value.
The conventional power control technique described above is strictly temporally-local and bimodal. If the received value of Ec/Nt (or Ec) during the immediate previous slot is larger than the set point, then a power down command is transmitted to the transmitter via a power control channel. If the received value of Ec/Nt (or Ec) during the immediate previous slot is smaller than the set point, then a power up command is transmitted to the transmitter via a power control channel. The transmitter then applies a predetermined power correction (sometimes also referred to as a step size) of a fixed value to compensate for the excess or deficient received power in a fading channel. For example, the transmitter may apply a step size of 0.5 or 1 decibels. The same step size is applied regardless of how far the received value of Ec/Nt (or Ec) deviates from the set point (i.e. the technique is bimodal). Furthermore, the value of the power control bit is strictly based on information associated with only the immediate previous slot (i.e. the technique is temporally-local).
Coded transmission using temporally-local and/or bimodal power control techniques may use more power than is needed to achieve the desired performance. The inefficiencies in the temporally-local and/or bimodal power control system may be caused by temporal channel variations in wireless telecommunication systems, insufficient granularity and/or step-size in the allowable levels of transmitted power, latencies in the power control loop, errors in the estimation of Ec/Nt, and the like. Consequently, closed loop power control may result in unnecessarily large system interference, system capacity waste, and reduced battery life.
The present invention is directed to addressing the effects of one or more of the problems set forth above.